In manufacturing structural elements such as metal machine parts, it is often necessary that the elements have a finished surface of particular design and shape. As an example, machine elements which are adapted to cooperate in a sliding, gearing or camming relation must have cooperating surfaces of precise shape. Where these surfaces are straight, circular, or some other common shape, the machining or surfacing is not too difficult. Where, however, the desired surface of the element is a complicated curve, as for example, one having an ever changing radius of curvature, the machining thereof becomes both difficult and expensive.
Where tolerances are critical, a grinding machine is usually employed. Grinding machines can produce extremely accurate surfaces; but when these surfaces are of unusual shape, the expense of constructing the machine to perform the particular grinding operation is often prohibitive. It may, for example, take a number of separate grinding operations to produce a particular complex surface with each of the grinding operations requiring that the workpiece be fed through a separate grinding machine. Also, with presently available grinding machines, the feeding of the workpiece relative to the grinding wheel is difficult to control both with regard to its direction of movement and rate of feed past the grinding wheel. Different sized grinding wheels generally require that the workpiece be fed through different paths in order to produce the same surface. Similarly, as the grinding wheel becomes worn during a grinding operation, adjustments in the direction of movement of the workpiece must be made in order to maintain the desired surface cut. This is especially true where a curved surface is desired. In addition to the problems encountered with different sized grinding wheels, any changes in rate of feed of the workpiece relative to the grinding wheel adversely affects both efficiency of operation and the quality of the finished surface. Different grinding rates produce different surface finishes. This, in turn, necessitates further processing of the workpiece to obtain uniformity.
One area which is particularly sensitive to the quality of the surface produced is in the machining of the stator cavity of a rotary combustion engine e.g. a Wankel engine. The cavity of such an engine has the contour of a peritrochoid which lies outside the theoretical trochoid of the engine by an amount equal to the apex radius of the rotor seal. Without a precision ground and finished peritroidal surface, wear caused by the internally driven rotor engaging against this surface through the apex seal occurs unacceptably fast. Also, the functioning of the engine is adversely affected by an improper mating of the rotor seal with the cavity wall surface of the stator due to inaccuracy in the geometric shape of the peritrochoid.
Several methods have been employed to control the relative movement between the work surface and the grinding tool so as to produce a true peritroichoid cavity contour. One means, revealed in the Baier U.S. Pat. No. 2,870,578, reveals a mechanism for reconstructing, through various spur gears and linkage arms, the theoretical trochoidal contour. The apparatus, although functional, is expensive and time consuming.
Another means for machining a peritrochoidal contour is revealed in an earlier U.S. Pat. No. 3,663,188 wherein I revealed use of a cam which linearly moved the spindle around which the workpiece was revolved. However, when a cam is so employed, a means for maintaining a normalty relation between the grinder at the contact point of the work surface and the contact point of the follower to the cam is required. In that patent, I used an annular cam with cam followers against both the inner and the outer cam surfaces to compound the gyration of the workpiece to maintain normalty between grinder and work surface. Other means to maintain normalty where a cam is employed include the use of spur gears. In grinding a complex contour, or for that matter, milling a complex profile, means must be provided for guiding the center of the cutter or the grinding wheel in a predetermined path. If, for any reason, the diameter of the cutter or the grinding wheel would change to a larger or smaller size, it would undercut the part in some places or leave stock on the part in other places and would not produce the correct profile, except if the profile were a true circle. To overcome this problem, a pivoting slide has been provided to allow the grinding spindle to be adjusted for different wheel sizes by rotating the slide so that the infeed takes place in the normal plane, as described above. On my U.S. Pat. No. 3,663,188, the infeed slide was passed through the center of a stationary plane, which intersected the center of the grinding spindle and the cam follower. In that machine, the cam and the part rotated together and adjusted themselves so that the contact point between the grinding wheel and the trochoid and the follower and the cam remained always on the stationary normal plane.
Where the stator housing to be machined is for a large engine which approaches or exceeds 1000 horse power per cavity and can weight several thousand pounds, alternate simpler machinery is required to properly machine the peretrochoidal contour. Attempts have been made to produce accurate peritrochoidal cavities for such very large rotary combustion engines by numerical controlled machines using rectangular coordinates. The results have been far from satisfactory.
In grinding contour profiles of any shape, whether cams, mechanical linkages, numerical control or gears are employed, it is necessary to provide for a normalty control so that, as the grinding wheel wears, the contour and size of the work piece remains unchanged. Cam-controlled machines have been designed with tapered followers which provide a means for adjustment as the grinding wheel wears.